This invention relates to circuits that electrostatically drive MicroElectroMechanical Systems (MEMS) structures, and more specifically to systems and methods that drive individual micromirrors in an array of micromirrors.
MEMS micromirrors have long been used for steering light beams in a variety of applications such as bar code scanners, image projectors, and optical networking. In all such applications, the micromirror element is actuated in response to an external stimulus provided by a controlling mechanism. One of the well-established and most-widely used methods of actuating the micromirrors is by electrostatic means. See, e.g., U.S. Pat. Nos. 4,317,611, 5,212,582, and 6,028,689. In this method, a drive electrode is electrically isolated from but placed in close proximity to a micromirror. The micromirror body is biased at a certain voltage potential and the drive electrode is biased at another potential level. The difference between the potential levels exerts an electrostatic force on the micromirror element and changes its position.
Micromirrors can be controlled in digital mode or in analog mode, depending on the requirements of the particular application. Digital control mode is shown, e.g., in U.S. Pat. No. 5,535,047, where the micromirror element is in one of the two stable positions in response to a digital control signal.
On the other hand, applications that need positioning of the mirror element at arbitrary intermediate points of its overall movement range require analog control mode of operation See, e.g., U.S. Pat. Nos. 4,441,791 and 6,028,689. The analog control mode is typically complemented by a closed control loop that measures the position of the micromirror element and provides corrective feedback to the driving circuitry.
It is desirable to build large arrays of micromirrors in a variety of applications. One such application is a spatial light modulator where the mirror array is used to reproduce an image on a projection screen by selectively steering beams from a uniform light source that shines on the array. Another application of the micromirror arrays is in the area of optical networking where individual light beams carrying digital data are steered by the micromirrors of the array for traffic routing in an xe2x80x9call opticalxe2x80x9d network. In the latter application, the size of the array that can feasibly be attained is an important parameter since it defines the volume of network traffic that an optical router can handle.
One of the problems associated with providing electrostatic drive to a large array of micromirrors is the prohibitively large number of control lines that are required to access the TV drive electrodes. A solution to this problem is a matrix addressing scheme such as the one disclosed in U.S. Pat. No. 4,441,791, where analog voltage levels are written to capacitors that serve as storage elements in a time multiplexed arrangement. A similar addressing scheme is also described in U.S. Pat. No. 4,271,488. Provided that the leakage currents are low and the update rate of the system can accommodate the over sampling level required by the control loop, the voltages on these holding capacitors can bias the drive electrodes. This method works satisfactorily when the accuracy requirement of the system is low, and the excitement voltage levels that the micromirrors require are suitable for making relatively simple analog switches. These analog switches may be integrated along with the micromirrors or integrated separately and assembled at a later stage in close proximity with the micromirrors.
However, when high voltages (e.g., defined as voltage levels on the order of a few tens of Volts to hundreds of Volts) and high accuracy levels (xcx9c14 bits) are required to excite the micromirrors, this approach is no longer suitable since it is difficult to make analog switches that can switch large voltage levels while satisfying various accuracy parameters such as off-isolation, crosstalk, and charge injection. Even if an ideal switch could be built and integrated within the array, the resulting system would require considerable external resources in digital-to-analog converters (DACs) and high voltage/high speed voltage buffers for setting up and delivering the precise analog voltage levels to the array. Moreover, these external resources would need to meet severe performance requirements. These constraints impose substantial limitations on the size of the micromirror arrays that can be built
Therefore, there is a need for electrostatic drivers for micromirror arrays that do not require high voltage analog switches or closely packed analog Fission lines.
In accordance with the present invention, a high voltage driver for a micromirror array is presented. In some embodiments, a High Voltage MEMS Driver (HVMD or xe2x80x9cMEMS driversxe2x80x9d) cell includes at least one High Voltage Digitally Controlled Integrator (HVDCI) cell which includes at least one programmable current source supplying an output current to an Integrate-And-Hold (AH) capacitor for a controlled duration to develop a voltage across the IAH capacitor. The voltage is applied to a drive electrode that electrostatically drives a MEMS structure. The HVDCI cell includes a digital control block generating digital control signals, a reference current source receiving the digital control signals and in response generating a reference current, and a high voltage output stage receiving the reference current and generating the output current to the IAH capacitor.
In some embodiments, the digital control block includes a counter coupled to a digital bus to receive an integration duration, and a number of storage elements (e.g., latches or registers) coupled to the digital bus to receive an integration direction and an integration current level. The counter outputs an active count signal to the reference current source during the integration duration. The latches output the integration direction and the integration current level to the reference current source.
In some embodiments, the reference current source includes a first current mirror with a reference branch and a number of output branches, and a decoder selectively enabling the output branches in response to the active count signal and the integration current level. In some embodiments, the first current mirror includes four output branches having respective current mirror ratios of 1:1, 1:1, 2:1, and 4:1.
In some embodiments, the reference current source further includes a second current mirror with a reference branch, a Rot group of output branches having a first common output node, and a second group of output branches having a second common output node. The decoder selectively enables the first or the second group of output branches in response to the active count signal, the integration direction, and the integration current level. In some embodiments, the first and the second group of output branches each include two output branches having respective current mirror ratios of 1/4:1 and 15/4:1.
In some embodiments, the high voltage output stage includes a pull-down path having a first current mirror and a second current mirror. The first current mirror includes a reference branch having an end coupled to a low voltage supply and another end coupled to the first common output node of the reference current source. The first current mirror fierier includes an output branch having an end coupled to the low voltage supply. The second current mirror includes a reference branch having an end coupled to the output branch of the first current mirror and another end coupled to ground. The second current mirror further includes an output branch having an end coupled to the IAH capacitor, and another end coupled to ground. In some embodiments, the first and the second output branches have respective current mirror ratios of 1:4 and 1:1. In some embodiments, the output branch of the second current mirror includes a high voltage transistor.
In some embodiments, the high voltage output stage further includes a pull-up path having a third current mirror, a fourth current minor, and a fifth current mirror. The third current mirror includes a reference branch having an end coupled to the low voltage supply and another end coupled to the second common output node of the reference current source. The third current mirror further includes an output branch having an end coupled to the low voltage supply. The fourth current mirror includes a reference branch having an end coupled to the output branch of the third current mirror, and another end coupled to ground The fourth current mirror further includes an output branch having an end coupled to ground. The fifth current mirror includes a reference branch having an end coupled to the output branch of the fourth current mirror and another end coupled to a high voltage supply. The fifth current mirror fur includes an output branch having an end coupled to the high voltage supply and another end coupled to the IAH capacitor. In some embodiments, each output branch of the fourth and fifth current mirrors includes a high voltage transistor coupled to the high voltage supply. In some embodiments, the third, the fourth, and the fifth mirror branches have respective current mirror ratios of 1:4, 1:1, and 1:1.
Embodiments of the MEMS driver described above may be manufactured in such a size that allows the HVMD cell to be integrated underneath a micromirror within an area of, e.g., 1.2 by 1.2 mm2.
Embodiments of the MEMS driver described above are beneficial in applications where the position of the micromirror is controlled by a digital control feedback loop. The digital control block takes as input a digital control word once every update cycle. The update cycle rate is defined by the oversampling (i.e., sampling analog signals at a great number of times per second to create an accurate conversion to digital signals) requirements of a digital control feedback loop. When a digital code word is loaded into the digital control block, the counter counts during the integration duration at a rate determined by the system clock frequency. The reference current source and the high voltage output stage are enabled during the count at a magnitude defined by the integration current level and in the integration direction. The high voltage output stage changes the voltage on the IAH capacitor by the precise amount as dictated by the digital control word. The digital control feedback loop keeps track of the position of the micromirror, thus the actual voltage level on the IAH capacitor need not be known.
Embodiments of the MEMS driver described above render external high performance digital-to-analog converters and high voltage amplifiers unnecessary, and make possible low voltage digital interface with external controllers. Eliminating the high performance DACs and amplifiers reduces overall power consumption of the MEMS driver. Since only digital signals are used to distribute the micromirror control information across an array of MEMS drivers, the MEMS driver array is inherently scalable with its size only limited by the speed of the digital logic. As the process technologies improve, and the speed of the digital logic increases, larger MEMS driver arrays and micromirror arrays can be manufactured
Embodiments of the MEMS driver described above are particularly beneficial for micromirrors that require high voltages (tens to hundreds of volts) for positioning. The MEMS driver may be formed on a substrate using widely available CMOS processes supporting high voltage MOS transistors. Such high voltage transistors typically use a drain construction with a high voltage drift region and are able to sustain high drain-to-source voltages.
In embodiments of the MEMS driver in an array, high voltage control line routing and high voltage analog multiplexing are not required. Multiplexing can be done in the digital domain and all the routing between an external control circuitry and the MEMS driver array and within the MEMS driver array is of low voltage digital type (with the exception of an optional common low voltage reference bias line described later). High voltages may be confined to global power routing regions, and to areas within each individual HVMD cell, which are relatively easy to accommodate by inserting shielding below such lines if and where necessitated by the underlying circuitry. Furthermore, high drive voltages for one micromirror are confined to the area immediately below that micromirror which helps to avoid crosstalk between unrelated micromirrors in the array. In addition, digital control data can be transferred to the MEMS driver array in a serial data stream and converted to parallel data by additional circuitry on the same substrate as the MEMS driver array, further reducing the number of interconnects between the MEMS driver array and the external control circuitry.
Embodiments of MEMS driver generate voltages with a dynamic range that extends to within hundreds of millivolts of the supply rails. By using variable integration durations and integration current levels, various drive voltages can be realized without excessive circuitry.
These and other embodiments are further discussed below with reference to the following figures.